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By Nancy B. Solomon, AIA

Water, water everywhere

Buildings get wet: Some building materials are made with water; others are rained on during construction. Roofs and windows leak. Pipes break. And moisture-laden air finds the path of least resistance. Despite such realities, too many architects and builders design and construct as if water will never enter the building.

This was not a problem years ago, when construction systems were more robust. Traditional materials, most of which are vapor permeable, installed according to traditional methods, could easily store a reasonable amount of moisture and allow it to gradually dissipate, as atmospheric or other environmental conditions changed, without damaging the building assembly. But as construction practices evolved over the 20th century, the balance of moisture and materials that we had come to take for granted began to change.

 

Mold grows under sheet vinyl covering a concrete floor in the Midwest because a vapor barrier had not been placed below the slab during construction, thus allowing ground moisture to penetrate the concrete. Photography: Courtesy Environmental Health & Engineering

Modern construction systems consist of many materials that are less permeable than traditional materials and so can neither store moisture vapor nor allow it to pass. Forensic engineer Joseph Lstiburek, a principal of Building Science Corporation in Westford, Massachusetts, estimates that, on average, the water-storage capacity of materials in a typical house has decreased from about 500 gallons a century ago to about 5 gallons today. And impermeable materials placed in the wrong location—like the vinyl wallpaper that has only too often been applied on the cooler interior walls of hotel rooms in hot, humid climates—can trap moisture where it doesn’t belong.

Newer, more processed materials—such as engineered woods and paper-faced gypsum board—offer mold a smorgasbord of more easily digestible food than do the traditional lumber and plaster that they replace. “To mold, plywood is like candy and paper is pablum,” says Lstiburek. So the now-wet paper on the gypsum board behind the vinyl wallpaper in that southern hotel room provides a veritable feast for the ever-present mold spores.